In a communication system, if an encryption algorithm is permitted by the network but is not supported by the plug-in card of the mobile station (MS), errors may occur in the encryption process.
For example, the general GSM/GPRS system uses a 64-bit encryption key. Although the 64-bit encryption key can provide certain security assurance, the security level is relatively low because the key length is short. Thus, a 128-bit encryption key is introduced in the prior art. The GSM system defines an A5/4 encryption algorithm for the 128-bit encryption key. Currently, the universal subscriber identity module (USIM) card can support the A5/4 algorithm. However, the subscriber identity module (SIM) card does not support the A5/4 encryption algorithm. For a MS whose plug-in card is a SIM card, the SIM card cannot support the A5/4 encryption algorithm. Thus, during the data interactions between the MS and the network, errors may occur in the encryption process if the A5/4 encryption algorithm is used.